In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a rotary coolant pump. Other venues also present situations where containment of a process fluid is critical, such as in the case of rotary chemical processing pumps or other equipment where leakage around a shaft is critical, such as a backup seal to packing in a valve.
A rotary pump such as a reactor coolant pump or chemical processing pump typically is a centrifugal pump designed to move large volumes of process fluid (e.g., reactor coolant) at a wide range of temperatures and pressures (including very high temperatures and pressures). Such a pump normally includes hydraulic, shaft seal and motor sections. A hydraulic section usually includes an impeller mounted at an end of a pump shaft which is operable within the pump casing to pump process fluid. A motor section includes a motor which is coupled to drive the pump shaft. A middle shaft seal section usually includes tandem sealing assemblies located concentric to, and near the top (motor end) end of, the pump shaft. Such sealing assemblies normally are configured for allowing but minimal process fluid leakage along the pump shaft during normal operating condition. Representative examples of known pump shaft sealing assemblies, at least in the context of reactor coolant pumps, may be found in the following U.S. Pat. Nos.: MacCrum (No. 3,522,948), Singleton (No. 3,529,838), Villasor (No. 3,632,117), Andrews et al (No. 3,720,222) and Boes (No. 4,275,891).
Pump shaft sealing assemblies, as such, must normally be capable of containing fluids at a high system pressure without excessive leakage. Tandem arrangements of sealing assemblies or mechanical seals, for instance, serve to break down the pressure in stages. Pump sealing assemblies in fact may act as controlled-leakage seals which, in operation, allow a minimal amount of controlled leakage at each stage while preventing excessive leakage of process fluid (e.g., reactor coolant) from the primary fluid system to respective seal leakoff ports. This applies in many scenarios where containment of excess leakage is critical. In the case of nuclear reactor coolant pumps, since pump sealing assemblies can be prone to failure, e.g. in response to unmitigated high temperatures of reactor coolant, any resultant excessive leakage rates could lead to reactor coolant uncovering of a reactor core, and subsequent core damage. (To be more precise, at least in nuclear reactor cooler pumps, seal packages tend to be isolated from the high temperature fluid in the vicinity of the impeller and other hydraulic components via the injection of a cool buffer fluid just upstream of the seal package, and/or by an auxiliary cooling system heat exchanger which cools the hot water flowing from the hot impeller/hydraulic part of the pump. The latter auxiliary cooling system can come into play if the aforementioned cool buffer fluid supply is lost. If both the buffer fluid supply and the cooling water supply to the auxiliary cooling system are lost the seal package can be challenged by exposure to high temperature water.)
While U.S. Pat. No. 5,171,024 (Janocko) discloses a shutdown seal arrangement for preventing and arresting excess fluid leakage along a pump shaft, needs continually are being recognized in connection with providing an even more effective arrangement, whether in the context of nuclear reactor coolant pumps or other contexts such as chemical processing pumps.